The charts take it year by year and then average each year together.

So, in order to calculate this seasonal pattern, the data for the whole time period is downloaded and then broken up into years.

Once you have each year seperated out, the weekly change is calculated each week during the year.

Once the weekly changes have been calculated, then the averages are calculated using all of the indivudual years.  

Finally, the index chart that you see is what you should expect to see each year. 

So, starting in the month of January, we expect to see a rally in corn until June and then a sell off into the fall.

Once the chart reaches the end of the year, the precess starts over.

In other words, these are not cumulative percentage changes.

I hope this makes sense.